auth-rsa.c - platform/external/openssh - Gitiles

 /* $OpenBSD: auth-rsa.c,v 1.67 2006/03/25 18:29:35 deraadt Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  *                    All rights reserved
  * RSA-based authentication.  This code determines whether to admit a login
  * based on RSA authentication.  This file also contains functions to check
  * validity of the host key.
  *
  * As far as I am concerned, the code I have written for this software
  * can be used freely for any purpose.  Any derived versions of this
  * software must be clearly marked as such, and if the derived work is
  * incompatible with the protocol description in the RFC file, it must be
  * called by a name other than "ssh" or "Secure Shell".
  */

 #include "includes.h"

 #include <sys/types.h>
 #include <sys/stat.h>

 #include <openssl/rsa.h>
 #include <openssl/md5.h>

 #include "rsa.h"
 #include "packet.h"
 #include "xmalloc.h"
 #include "ssh1.h"
 #include "uidswap.h"
 #include "match.h"
 #include "auth-options.h"
 #include "pathnames.h"
 #include "log.h"
 #include "servconf.h"
 #include "auth.h"
 #include "hostfile.h"
 #include "monitor_wrap.h"
 #include "ssh.h"
 #include "misc.h"

 /* import */
 extern ServerOptions options;

 /*
  * Session identifier that is used to bind key exchange and authentication
  * responses to a particular session.
  */
 extern u_char session_id[16];

 /*
  * The .ssh/authorized_keys file contains public keys, one per line, in the
  * following format:
  *   options bits e n comment
  * where bits, e and n are decimal numbers,
  * and comment is any string of characters up to newline.  The maximum
  * length of a line is SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
  * description of the options.
  */

 BIGNUM *
 auth_rsa_generate_challenge(Key *key)
 {
 	BIGNUM *challenge;
 	BN_CTX *ctx;

 	if ((challenge = BN_new()) == NULL)
 		fatal("auth_rsa_generate_challenge: BN_new() failed");
 	/* Generate a random challenge. */
 	BN_rand(challenge, 256, 0, 0);
 	if ((ctx = BN_CTX_new()) == NULL)
 		fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
 	BN_mod(challenge, challenge, key->rsa->n, ctx);
 	BN_CTX_free(ctx);

 	return challenge;
 }

 int
 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
 {
 	u_char buf[32], mdbuf[16];
 	MD5_CTX md;
 	int len;

 	/* don't allow short keys */
 	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
 		error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
 		    BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
 		return (0);
 	}

 	/* The response is MD5 of decrypted challenge plus session id. */
 	len = BN_num_bytes(challenge);
 	if (len <= 0 || len > 32)
 		fatal("auth_rsa_verify_response: bad challenge length %d", len);
 	memset(buf, 0, 32);
 	BN_bn2bin(challenge, buf + 32 - len);
 	MD5_Init(&md);
 	MD5_Update(&md, buf, 32);
 	MD5_Update(&md, session_id, 16);
 	MD5_Final(mdbuf, &md);

 	/* Verify that the response is the original challenge. */
 	if (memcmp(response, mdbuf, 16) != 0) {
 		/* Wrong answer. */
 		return (0);
 	}
 	/* Correct answer. */
 	return (1);
 }

 /*
  * Performs the RSA authentication challenge-response dialog with the client,
  * and returns true (non-zero) if the client gave the correct answer to
  * our challenge; returns zero if the client gives a wrong answer.
  */

 int
 auth_rsa_challenge_dialog(Key *key)
 {
 	BIGNUM *challenge, *encrypted_challenge;
 	u_char response[16];
 	int i, success;

 	if ((encrypted_challenge = BN_new()) == NULL)
 		fatal("auth_rsa_challenge_dialog: BN_new() failed");

 	challenge = PRIVSEP(auth_rsa_generate_challenge(key));

 	/* Encrypt the challenge with the public key. */
 	rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);

 	/* Send the encrypted challenge to the client. */
 	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
 	packet_put_bignum(encrypted_challenge);
 	packet_send();
 	BN_clear_free(encrypted_challenge);
 	packet_write_wait();

 	/* Wait for a response. */
 	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
 	for (i = 0; i < 16; i++)
 		response[i] = (u_char)packet_get_char();
 	packet_check_eom();

 	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
 	BN_clear_free(challenge);
 	return (success);
 }

 /*
  * check if there's user key matching client_n,
  * return key if login is allowed, NULL otherwise
  */

 int
 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
 {
 	char line[SSH_MAX_PUBKEY_BYTES], *file;
 	int allowed = 0;
 	u_int bits;
 	FILE *f;
 	u_long linenum = 0;
 	struct stat st;
 	Key *key;

 	/* Temporarily use the user's uid. */
 	temporarily_use_uid(pw);

 	/* The authorized keys. */
 	file = authorized_keys_file(pw);
 	debug("trying public RSA key file %s", file);

 	/* Fail quietly if file does not exist */
 	if (stat(file, &st) < 0) {
 		/* Restore the privileged uid. */
 		restore_uid();
 		xfree(file);
 		return (0);
 	}
 	/* Open the file containing the authorized keys. */
 	f = fopen(file, "r");
 	if (!f) {
 		/* Restore the privileged uid. */
 		restore_uid();
 		xfree(file);
 		return (0);
 	}
 	if (options.strict_modes &&
 	    secure_filename(f, file, pw, line, sizeof(line)) != 0) {
 		xfree(file);
 		fclose(f);
 		logit("Authentication refused: %s", line);
 		restore_uid();
 		return (0);
 	}

 	/* Flag indicating whether the key is allowed. */
 	allowed = 0;

 	key = key_new(KEY_RSA1);

 	/*
 	 * Go though the accepted keys, looking for the current key.  If
 	 * found, perform a challenge-response dialog to verify that the
 	 * user really has the corresponding private key.
 	 */
 	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
 		char *cp;
 		char *key_options;
 		int keybits;

 		/* Skip leading whitespace, empty and comment lines. */
 		for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
 			;
 		if (!*cp || *cp == '\n' || *cp == '#')
 			continue;

 		/*
 		 * Check if there are options for this key, and if so,
 		 * save their starting address and skip the option part
 		 * for now.  If there are no options, set the starting
 		 * address to NULL.
 		 */
 		if (*cp < '0' || *cp > '9') {
 			int quoted = 0;
 			key_options = cp;
 			for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
 				if (*cp == '\\' && cp[1] == '"')
 					cp++;	/* Skip both */
 				else if (*cp == '"')
 					quoted = !quoted;
 			}
 		} else
 			key_options = NULL;

 		/* Parse the key from the line. */
 		if (hostfile_read_key(&cp, &bits, key) == 0) {
 			debug("%.100s, line %lu: non ssh1 key syntax",
 			    file, linenum);
 			continue;
 		}
 		/* cp now points to the comment part. */

 		/* Check if the we have found the desired key (identified by its modulus). */
 		if (BN_cmp(key->rsa->n, client_n) != 0)
 			continue;

 		/* check the real bits  */
 		keybits = BN_num_bits(key->rsa->n);
 		if (keybits < 0 || bits != (u_int)keybits)
 			logit("Warning: %s, line %lu: keysize mismatch: "
 			    "actual %d vs. announced %d.",
 			    file, linenum, BN_num_bits(key->rsa->n), bits);

 		/* We have found the desired key. */
 		/*
 		 * If our options do not allow this key to be used,
 		 * do not send challenge.
 		 */
 		if (!auth_parse_options(pw, key_options, file, linenum))
 			continue;

 		/* break out, this key is allowed */
 		allowed = 1;
 		break;
 	}

 	/* Restore the privileged uid. */
 	restore_uid();

 	/* Close the file. */
 	xfree(file);
 	fclose(f);

 	/* return key if allowed */
 	if (allowed && rkey != NULL)
 		*rkey = key;
 	else
 		key_free(key);
 	return (allowed);
 }

 /*
  * Performs the RSA authentication dialog with the client.  This returns
  * 0 if the client could not be authenticated, and 1 if authentication was
  * successful.  This may exit if there is a serious protocol violation.
  */
 int
 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
 {
 	Key *key;
 	char *fp;
 	struct passwd *pw = authctxt->pw;

 	/* no user given */
 	if (!authctxt->valid)
 		return 0;

 	if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
 		auth_clear_options();
 		return (0);
 	}

 	/* Perform the challenge-response dialog for this key. */
 	if (!auth_rsa_challenge_dialog(key)) {
 		/* Wrong response. */
 		verbose("Wrong response to RSA authentication challenge.");
 		packet_send_debug("Wrong response to RSA authentication challenge.");
 		/*
 		 * Break out of the loop. Otherwise we might send
 		 * another challenge and break the protocol.
 		 */
 		key_free(key);
 		return (0);
 	}
 	/*
 	 * Correct response.  The client has been successfully
 	 * authenticated. Note that we have not yet processed the
 	 * options; this will be reset if the options cause the
 	 * authentication to be rejected.
 	 */
 	fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
 	verbose("Found matching %s key: %s",
 	    key_type(key), fp);
 	xfree(fp);
 	key_free(key);

 	packet_send_debug("RSA authentication accepted.");
 	return (1);
 }
	/* $OpenBSD: auth-rsa.c,v 1.67 2006/03/25 18:29:35 deraadt Exp $ */
	/*
	* Author: Tatu Ylonen <ylo@cs.hut.fi>
	* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
	* All rights reserved
	* RSA-based authentication. This code determines whether to admit a login
	* based on RSA authentication. This file also contains functions to check
	* validity of the host key.
	*
	* As far as I am concerned, the code I have written for this software
	* can be used freely for any purpose. Any derived versions of this
	* software must be clearly marked as such, and if the derived work is
	* incompatible with the protocol description in the RFC file, it must be
	* called by a name other than "ssh" or "Secure Shell".
	*/

	#include "includes.h"

	#include <sys/types.h>
	#include <sys/stat.h>

	#include <openssl/rsa.h>
	#include <openssl/md5.h>

	#include "rsa.h"
	#include "packet.h"
	#include "xmalloc.h"
	#include "ssh1.h"
	#include "uidswap.h"
	#include "match.h"
	#include "auth-options.h"
	#include "pathnames.h"
	#include "log.h"
	#include "servconf.h"
	#include "auth.h"
	#include "hostfile.h"
	#include "monitor_wrap.h"
	#include "ssh.h"
	#include "misc.h"

	/* import */
	extern ServerOptions options;

	/*
	* Session identifier that is used to bind key exchange and authentication
	* responses to a particular session.
	*/
	extern u_char session_id[16];

	/*
	* The .ssh/authorized_keys file contains public keys, one per line, in the
	* following format:
	* options bits e n comment
	* where bits, e and n are decimal numbers,
	* and comment is any string of characters up to newline. The maximum
	* length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
	* description of the options.
	*/

	BIGNUM *
	auth_rsa_generate_challenge(Key *key)
	{
	BIGNUM *challenge;
	BN_CTX *ctx;

	if ((challenge = BN_new()) == NULL)
	fatal("auth_rsa_generate_challenge: BN_new() failed");
	/* Generate a random challenge. */
	BN_rand(challenge, 256, 0, 0);
	if ((ctx = BN_CTX_new()) == NULL)
	fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
	BN_mod(challenge, challenge, key->rsa->n, ctx);
	BN_CTX_free(ctx);

	return challenge;
	}

	int
	auth_rsa_verify_response(Key key, BIGNUM challenge, u_char response[16])
	{
	u_char buf[32], mdbuf[16];
	MD5_CTX md;
	int len;

	/* don't allow short keys */
	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
	error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
	BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
	return (0);
	}

	/* The response is MD5 of decrypted challenge plus session id. */
	len = BN_num_bytes(challenge);
	if (len <= 0 \|\| len > 32)
	fatal("auth_rsa_verify_response: bad challenge length %d", len);
	memset(buf, 0, 32);
	BN_bn2bin(challenge, buf + 32 - len);
	MD5_Init(&md);
	MD5_Update(&md, buf, 32);
	MD5_Update(&md, session_id, 16);
	MD5_Final(mdbuf, &md);

	/* Verify that the response is the original challenge. */
	if (memcmp(response, mdbuf, 16) != 0) {
	/* Wrong answer. */
	return (0);
	}
	/* Correct answer. */
	return (1);
	}

	/*
	* Performs the RSA authentication challenge-response dialog with the client,
	* and returns true (non-zero) if the client gave the correct answer to
	* our challenge; returns zero if the client gives a wrong answer.
	*/

	int
	auth_rsa_challenge_dialog(Key *key)
	{
	BIGNUM challenge, encrypted_challenge;
	u_char response[16];
	int i, success;

	if ((encrypted_challenge = BN_new()) == NULL)
	fatal("auth_rsa_challenge_dialog: BN_new() failed");

	challenge = PRIVSEP(auth_rsa_generate_challenge(key));

	/* Encrypt the challenge with the public key. */
	rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);

	/* Send the encrypted challenge to the client. */
	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
	packet_put_bignum(encrypted_challenge);
	packet_send();
	BN_clear_free(encrypted_challenge);
	packet_write_wait();

	/* Wait for a response. */
	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
	for (i = 0; i < 16; i++)
	response[i] = (u_char)packet_get_char();
	packet_check_eom();

	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
	BN_clear_free(challenge);
	return (success);
	}

	/*
	* check if there's user key matching client_n,
	* return key if login is allowed, NULL otherwise
	*/

	int
	auth_rsa_key_allowed(struct passwd pw, BIGNUM client_n, Key **rkey)
	{
	char line[SSH_MAX_PUBKEY_BYTES], *file;
	int allowed = 0;
	u_int bits;
	FILE *f;
	u_long linenum = 0;
	struct stat st;
	Key *key;

	/* Temporarily use the user's uid. */
	temporarily_use_uid(pw);

	/* The authorized keys. */
	file = authorized_keys_file(pw);
	debug("trying public RSA key file %s", file);

	/* Fail quietly if file does not exist */
	if (stat(file, &st) < 0) {
	/* Restore the privileged uid. */
	restore_uid();
	xfree(file);
	return (0);
	}
	/* Open the file containing the authorized keys. */
	f = fopen(file, "r");
	if (!f) {
	/* Restore the privileged uid. */
	restore_uid();
	xfree(file);
	return (0);
	}
	if (options.strict_modes &&
	secure_filename(f, file, pw, line, sizeof(line)) != 0) {
	xfree(file);
	fclose(f);
	logit("Authentication refused: %s", line);
	restore_uid();
	return (0);
	}

	/* Flag indicating whether the key is allowed. */
	allowed = 0;

	key = key_new(KEY_RSA1);

	/*
	* Go though the accepted keys, looking for the current key. If
	* found, perform a challenge-response dialog to verify that the
	* user really has the corresponding private key.
	*/
	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
	char *cp;
	char *key_options;
	int keybits;

	/* Skip leading whitespace, empty and comment lines. */
	for (cp = line; cp == ' ' \|\| cp == '\t'; cp++)
	;
	if (!cp \|\| cp == '\n' \|\| *cp == '#')
	continue;

	/*
	* Check if there are options for this key, and if so,
	* save their starting address and skip the option part
	* for now. If there are no options, set the starting
	* address to NULL.
	*/
	if (cp < '0' \|\| cp > '9') {
	int quoted = 0;
	key_options = cp;
	for (; cp && (quoted \|\| (cp != ' ' && *cp != '\t')); cp++) {
	if (*cp == '\\' && cp[1] == '"')
	cp++; /* Skip both */
	else if (*cp == '"')
	quoted = !quoted;
	}
	} else
	key_options = NULL;

	/* Parse the key from the line. */
	if (hostfile_read_key(&cp, &bits, key) == 0) {
	debug("%.100s, line %lu: non ssh1 key syntax",
	file, linenum);
	continue;
	}
	/* cp now points to the comment part. */

	/* Check if the we have found the desired key (identified by its modulus). */
	if (BN_cmp(key->rsa->n, client_n) != 0)
	continue;

	/* check the real bits */
	keybits = BN_num_bits(key->rsa->n);
	if (keybits < 0 \|\| bits != (u_int)keybits)
	logit("Warning: %s, line %lu: keysize mismatch: "
	"actual %d vs. announced %d.",
	file, linenum, BN_num_bits(key->rsa->n), bits);

	/* We have found the desired key. */
	/*
	* If our options do not allow this key to be used,
	* do not send challenge.
	*/
	if (!auth_parse_options(pw, key_options, file, linenum))
	continue;

	/* break out, this key is allowed */
	allowed = 1;
	break;
	}

	/* Restore the privileged uid. */
	restore_uid();

	/* Close the file. */
	xfree(file);
	fclose(f);

	/* return key if allowed */
	if (allowed && rkey != NULL)
	*rkey = key;
	else
	key_free(key);
	return (allowed);
	}

	/*
	* Performs the RSA authentication dialog with the client. This returns
	* 0 if the client could not be authenticated, and 1 if authentication was
	* successful. This may exit if there is a serious protocol violation.
	*/
	int
	auth_rsa(Authctxt authctxt, BIGNUM client_n)
	{
	Key *key;
	char *fp;
	struct passwd *pw = authctxt->pw;

	/* no user given */
	if (!authctxt->valid)
	return 0;

	if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
	auth_clear_options();
	return (0);
	}

	/* Perform the challenge-response dialog for this key. */
	if (!auth_rsa_challenge_dialog(key)) {
	/* Wrong response. */
	verbose("Wrong response to RSA authentication challenge.");
	packet_send_debug("Wrong response to RSA authentication challenge.");
	/*
	* Break out of the loop. Otherwise we might send
	* another challenge and break the protocol.
	*/
	key_free(key);
	return (0);
	}
	/*
	* Correct response. The client has been successfully
	* authenticated. Note that we have not yet processed the
	* options; this will be reset if the options cause the
	* authentication to be rejected.
	*/
	fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
	verbose("Found matching %s key: %s",
	key_type(key), fp);
	xfree(fp);
	key_free(key);

	packet_send_debug("RSA authentication accepted.");
	return (1);
	}